Small organic molecule based flow battery

ABSTRACT

The invention provides an electrochemical cell based on a new chemistry for a flow battery for large scale, e.g., grid-scale, electrical energy storage. Electrical energy is stored chemically at an electrochemical electrode by the protonation of small organic molecules called quinones to hydroquinones. The proton is provided by a complementary electrochemical reaction at the other electrode. These reactions are reversed to deliver electrical energy. A flow battery based on this concept can operate as a closed system. The flow battery architecture has scaling advantages over solid electrode batteries for large scale energy storage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority to U.S. ProvisionalApplication Nos. 61/705,845, filed Sep. 26, 2012, 61/823,258, filed May14, 2013, and 61/838,589, filed Jun. 24, 2013, each of which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under grant number0670-4322 from the Advanced Research Projects Agency-Energy-U.S.Department of Energy. The government has certain rights to theinvention.

BACKGROUND OF THE INVENTION

Intermittent renewable electrical power sources such as wind andphotovoltaics (PV) cannot replace a significant fraction of our currentfossil fuel-based electrical generation unless the intermittency problemis solved. Fluctuations in renewable source power are generally backedup by natural gas fired “peaker” plants. Inexpensive, reliable energystorage at or near the generation site could render the renewable sourcedispatchable (e.g. demand-following) and permit the gas peakers toreplace baseload coal. It could also permit full utilization of thetransmission capacity of power lines from the generation site,permitting supply capacity expansion while deferring the need fortransmission capacity expansion.

The advantages of flow batteries are giving them increased attention forgrid-scale electrical storage [1]: because all of the reactants andproducts are stored in tanks outside the electrochemical conversiondevice, the device itself may be optimized for the required power whilethe required energy is independently determined by the mass of reactantand the size of storage tanks. This can drive down the storage cost perkWh, which is the single most challenging requirement for grid-scalestorage. In contrast, in solid electrode batteries the energy/powerratio (i.e. the peak-power discharge time) does not scale and isinadequate for rendering intermittent renewable power sourcesdispatchable. Most solid-electrode batteries have peak-power dischargetimes <<1 hr., whereas rendering PV and wind dispatchable require ˜15and −50 hrs., respectively [2].

The commonly recognized technology options for grid-scale electricalenergy storage are summarized in Table 1. Commercial activity withzinc-bromine hybrid flow batteries illustrates the technical feasibilityof liquid bromine and hydrobromic acid as reactants. However, by itsnature the design—involving Zn plating within the electrochemicalconversion device—does not permit flow battery-like energy scaling; italso presents a dendrite-shorting risk [1]. Arguably the most developedflow battery technologies are vanadium redox flow batteries (VRBs) andsodium-sulfur batteries (NaSBs). Costs per kW are comparable, whereasVRBs are considerably more costly on a cost per kWh basis, in part dueto the high price of vanadium, which sets a floor on the ultimate costper kWh of a VRB [3]. The vanadium itself costs around $160/kWh based onrecent costs for V₂O₅ [4]. VRBs do benefit from a longer cycle life,with the ability to be cycled in excess of 10,000 times, whereas NaSBsare typically limited to about 4,500 cycles [3]. For VRBs, costs per kWare likely to move lower, as recent improvements in VRB cell design haveled to significantly higher power densities and current densities, withvalues of 0.55 W/cm² and 0.9 A/cm², respectively [5], but these don'thelp lower the ultimate floor on the cost per kWh. These values, to ourknowledge, represent the best performance achieved in VRBs reported todate in the literature. NaSBs have to operate above 300° C. to keep thereactants molten, which sets a floor on their operating costs. Over 100MW of NaSBs have been installed on the grid in Japan, but this is due togovernment fiat rather than market forces. NaSBs have the longestduration (energy/power) at ˜7 hrs. VRBs are the subject of aggressivedevelopment, whereas NaSBs represent a static target. There is alsorecent work on the regenerative electrolysis of hydrohalic acid todihalogen and dihydrogen [6-9], where the halogen is chlorine orbromine. These systems have the potential for lower storage cost per kWhthan VRBs due to the lower cost of the chemical reactants.

TABLE 1 Energy Storage for the grid. From Dunn et al. [3]; originalsource EPRI. Technology Capacity Power Duration % Efficiency Total costCost option Maturity (MWh) (MW) (hours) (total cycles) ($/kW) ($/kWh)CAES Demo 250 50  5 (>10,000) 1950-2150 390-430 (aboveground) AdvancedDemo 3.2-4.8 1-12 3.2-4   75-90 2000-4600  625-1150 Pb-acid   (4500)Na/S Commercial 7.2 1 7.2    75 3200-4000 445-555   (4500) Zn/Br flowDemo  5-50 1-10 5 60-65 1670-2015  340-1350 (>10,000) V redox Demo  4-401-10 4 65-70 3000-3310 750-830 (>10,000) Fe/Cr flow R&D 4 1 4    751200-1600 300-400 (>10,000) Zn/air R&D 5.4 1 5.4    75 1750-1900 325-350  (4500) Li ion Demo  4-24 1-10 2-4 90-94 1800-4100  900-1700   (4500)

SUMMARY OF THE INVENTION

The invention provides an electrochemical cell based on a new chemistryfor a flow battery for large scale, e.g., gridscale, electrical energystorage. Electrical energy is stored chemically at an electrochemicalelectrode by the protonation of small organic molecules called quinonesto hydroquinones. The proton is provided by a complementaryelectrochemical reaction at the other electrode. These reactions arereversed to deliver electrical energy. A flow battery based on thisconcept can operate as a closed system. The flow battery architecturehas scaling advantages over solid electrode batteries for large scaleenergy storage. Because quinone-to-hydroquinone cycling occurs rapidlyand reversibly in photosynthesis, we expect to be able to employ it toobtain high current density, high efficiency, and long lifetime in aflow battery. High current density drives down power-related costs. Theother advantages this particular technology would have over other flowbatteries include inexpensive chemicals, energy storage in the form ofsafer liquids, an inexpensive separator, little or no precious metalsusage in the electrodes, and other components made of plastic orinexpensive metals with coatings proven to afford corrosion protection.

Variations of a quinone-based cell are described. One is aquinone/hydroquinone couple as the negative electrode against a positiveelectrode with a redox active species. In one embodiment, the positiveelectrode and the negative electrode are quinone/hydroquinone couples.

In one aspect, the invention provides a rechargeable battery havingfirst and second electrodes, wherein in its charged state, the batteryincludes a redox active species in contact with the first electrode anda hydroquinone dissolved or suspended in aqueous solution in contactwith the second electrode, wherein during discharge the redox activespecies is reduced and the hydroquinone is oxidized to a quinone. Incertain embodiments, the redox active species is dissolved or suspendedin aqueous solution. Redox active species may include chlorine, bromine,iodine, oxygen, vanadium, chromium, cobalt, iron, manganese, cobalt,nickel, copper, or lead, in particular, bromine or a manganese oxide, acobalt oxide or a lead oxide. Alternatively, the redox active species isa second quinone dissolved or suspended in aqueous solution, asdescribed herein. In a specific embodiment, the hydroquinone andquinone, e.g., a water-soluble anthraquinone optionally including one ormore sulfonate groups, have a standard electrochemical potential below0.4 volts with respect to a standard hydrogen electrode. Typically, thefirst electrode is separated from the second electrode by a barrier thatinhibits the passage of the redox-active species and the hydroquinone,e.g., an ion conducting membrane or a size exclusion membrane. In aspecific embodiment, the first and second electrodes are separated by anion conducting barrier, and the redox active species includes bromine.

In another aspect, the invention features a rechargeable batteryincluding first and second electrodes separated by an ion conductinghydrocarbon barrier or size-exclusion barrier, wherein in its chargedstate, the battery includes a quinone at the first electrode and ahydroquinone at the second electrode, wherein during discharge, thequinone is reduced, and the hydroquinone is oxidized.

In a related aspect the invention features a rechargeable batteryincluding first and second electrodes separated by an ion conductingbarrier, wherein in its charged state, the battery includes a quinone inaqueous solution at the first electrode and a hydroquinone in aqueoussolution at the second electrode, wherein during discharge, the quinoneis reduced, and the hydroquinone is oxidized. In a further relatedaspect, the invention features a rechargeable battery including firstand second electrodes separated by an ion conducting barrier, wherein inits charged state, the battery includes bromine at the first electrodeand a hydroquinone at the second electrode, wherein during discharge,bromine is reduced, and the hydroquinone is oxidized. In yet a furtheraspect, the invention features a rechargeable battery including firstand second electrodes separated by an ion conducting hydrocarbonbarrier, wherein in its charged state, the battery includes a quinone atthe first electrode and a hydroquinone at the second electrode, whereinduring discharge, the quinone is reduced, and the hydroquinone isoxidized.

For any of these aspects, the quinone or hydroquinone in oxidized formis, for example, of formula (I) or (II):

wherein each of R₁-R₄ is independently selected from H, C₁₋₆ alkyl,halo, hydroxy, C₁₋₆ alkoxy, and SO₃H, or an ion thereof, e.g., H, C₁₋₆alkyl, halo, C₁₋₆ alkoxy, and SO₃H, or an ion thereof or H, C₁₋₆ alkyl,C₁₋₆ alkoxy, and SO₃H, or an ion thereof. In another embodiment, thequinone or hydroquinone in oxidized form is, for example, of formula(III):

wherein each of R₁-R₈ is independently selected from H, C₁₋₆ alkyl,halo, hydroxyl, C₁₋₆ alkoxy, and SO₃H, or an ion thereof, e.g., H, C₁₋₆alkyl, halo, C₁₋₆ alkoxy, and SO₃H, or an ion thereof, or H, C₁₋₆ alkyl,C₁₋₆ alkoxy, and SO₃H, or an ion thereof.

A rechargeable battery of the invention may further include a reservoirfor quinone and/or hydroquinone dissolved or suspended in aqueoussolution and a mechanism to circulate quinone and/or hydroquinone. Inparticular embodiments, the rechargeable battery is a flow battery.

Exemplary quinones or hydroquinones in oxidized form are of formula(A)-(D):

wherein each of R₁-R₁₀ is independently selected from H, optionallysubstituted C₁₋₆ alkyl, halo, hydroxy, optionally substituted C₁₋₆alkoxy, SO₃H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo,or an ion thereof, provided that two of R₁-R₆ for formula (A) are oxo,two or four of R₁-R₈ for formula (B) are oxo, and two, four, or six ofR₁-R₁₀ for formulas (C) and (D) are oxo, wherein the dashed linesindicate that the monocylic ring of formula (A), the bicyclic ring offormula (B), and the tricyclic rings of formulas (C) and (D) are fullyconjugated. In specific embodiments, R₁-R₁₀ is independently selectedfrom H, optionally substituted C₁₋₆ alkyl, hydroxy, optionallysubstituted C₁₋₆ alkoxy, SO₃H, amino, nitro, carboxyl, phosphoryl,phosphonyl, and oxo, or an ion thereof.

Exemplary quinones or hydroquinones in oxidized form may also be offormula (I)-(IX):

wherein each of R₁-R₈ is independently selected from H, optionallysubstituted C₁₋₆ alkyl, halo, hydroxy, optionally substituted C₁₋₆alkoxy, SO₃H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo,or an ion thereof. In particular embodiments, each of R₁-R₈ isindependently selected from H, optionally substituted C₁₋₆ alkyl,hydroxy, optionally substituted C₁₋₆ alkoxy, SO₃H, amino, nitro,carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof.

Specific quinones or hydroquinones in oxidized form for use with anyaspect of the invention include 9,10-anthraquinone-2,7-disulfonic acid,9,10-anthraquinone-2,6-disulfonic acid,9,10-anthraquinone-1,8-disulfonic acid,9,10-anthraquinone-1,5-disulfonic acid, 9,10-anthraquinone-2-sulfonicacid, 9,10-anthraquinone-2,3-dimethanesulfonic acid,1,8-dihydroxy-9,10-anthraquinone-2,7-disulfonic acid,1,5-dihydroxy-9,10-anthraquinone-2,6-disulfonic acid,1,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid,1,3,4-trihydroxy-9,10-anthraquinone-2-sulfonic acid,1,2-naphthoquinone-4-sulfonic acid, 1,4-naphthoquinone-2-sulfonic acid,2-chloro-1,4-naphthoquinone-3-sulfonic acid,2-bromo-1,4-naphthoquinone-3-sulfonic acid, or a mixture thereof.Further preferred quinones or hydroquinones in oxidized form include9,10-anthraquinone-2,7-disulfonic acid,9,10-anthraquinone-2,6-disulfonic acid,9,10-anthraquinone-1,8-disulfonic acid,9,10-anthraquinone-1,5-disulfonic acid, 9,10-anthraquinone-2-sulfonicacid, or a mixture thereof. An exemplary quinone for use with any aspectof the invention is 9,10-anthraquinone-2,7-disulfonate.

Additional quinones or hydroquinones in oxidized form include2-hydroxy-1,4-naphthoquinone-3-sulfonic acid,1,2,4-trihydroxybenzene-3-sulfonic acid,2,4,5-trihydroxybenzene-1,3-disulfonic acid2,3,5-trihydroxybenzene-1,4-disulfonic acid,2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid,2,3,5-trihydroxybenzene-1,4-disulfonic acid,2,3,5,6-tetrahydroxybenzene-1,4-disulfonic acid, or a mixture thereof.

Still further quinones and hydroquinones in oxidized form for use aloneor in mixtures in any aspect of the invention are described herein,e.g., in Table 4.

The invention also provides methods for storing electrical energy byapplying a voltage across the first and second electrodes and chargingany battery of the invention.

The invention also provides methods for providing electrical energy byconnecting a load to the first and second electrodes and allowing anybattery of the invention to discharge.

In certain embodiments, 4,5-dihydroxy-1,3-benzenedisulfonate and/or2,5-dihydroxy-benzenedisulfonate are specifically excluded as thehydroquinone or quinone in reduced form for any aspect of the invention.

The absence of active metal components in both redox chemistry andcatalysis represents a significant shift away from modern batteries. Inparticular, the use of quinones, such as9,10-anthraquinone-2,7-disulfonate, offers several advantages overcurrent flow battery technologies:

-   -   (1) Scalability: it contains the earth-abundant atoms, such as        carbon, sulfur, hydrogen and oxygen, and can be inexpensively        manufactured on large scales. Because some quinones are natural        products, there is also the possibility that the electrolyte        material can be renewably sourced.    -   (2) Kinetics: it undergoes rapid two-electron redox on simple        carbon electrodes and does not require a costly precious metal        catalyst.    -   (3) Stability: the quinone should exhibit minimal membrane        crossover because of its relatively large size and potential for        a dianionic state. Furthermore, although bromine crossover is a        known issue in zinc-bromine and hydrogen-bromine cells,        9,10-anthraquinone-2,7-disulfonate is stable to prolonged        heating in concentrated Br₂/HBr mixtures.    -   (4) Solubility: it has a solubility of order 1 M at pH 0 and can        be stored at relatively high energy densities.    -   (5) Tunability: The reduction potential and solubility of        quinones can be further optimized by introduction of        electron-donating functional groups such as —OH.        These features lower the capital cost of storage chemicals per        kWh, which sets a floor on the ultimate system cost per kWh at        any scale. Sulfonated anthraquinones are used on an industrial        scale in wood pulp processing for paper, and they can be readily        synthesized from the commodity chemicals anthraquinone and        oleum. We estimate chemical costs of $21 kWh⁻¹ for        9,10-anthraquinone-2,7-disulfonate and $6 kWh⁻¹ for bromine. A        quinone-bromine flow battery offers significant cost        improvements to vanadium flow batteries that cost $81 kWh⁻¹.        Optimization of engineering and operating parameters such as the        flow field geometry, electrode design, membrane separator, and        temperature should lead to significant performance improvements        in the future, as it has for vanadium flow batteries, which took        many years to surpass 100 mW cm⁻². The use of quinones        represents a new and promising direction for cost-effective,        large-scale energy storage.

For the purposes of this invention, the term “quinone” includes acompound having one or more conjugated, C₃₋₁₀ carbocyclic, fused rings,substituted, in oxidized form, with two or more oxo groups, which are inconjugation with the one or more conjugated rings. Preferably, thenumber of rings is from one to ten, e.g., one, two, or three, and eachring has 6 members.

By alkyl is meant straight chain or branched saturated groups from 1 to6 carbons. Alkyl groups are exemplified by methyl, ethyl, n- andiso-propyl, n-, sec-, iso- and tert-butyl, neopentyl, and the like, andmay be optionally substituted with one, two, three, or, in the case ofalkyl groups of two carbons or more, four substituents independentlyselected from the group consisting of halo, hydroxyl, C₁₋₆ alkoxy, SO₃H,amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo, or an ionthereof.

By “alkoxy” is meant a group of formula —OR, wherein R is an alkylgroup, as defined herein.

By “halo” is meant, fluoro, chloro, bromo, or iodo.

By “hydroxyl” is meant —OH.

By “amino” is meant —NH₂. An exemplary ion of amino is —NH₃ ⁺.

By “nitro” is meant —NO₂.

By “carboxyl” is meant —COOH. An exemplary ion of carboxyl, is —COO⁻.

By “sulfonyl” is meant —SO₃H. An exemplary ion of sulfonyl is —SO₃ ⁻.

By “phosphoryl” is meant —PO₃H₂. Exemplary ions of phosphoryl are —PO₃11and —PO₃ ²⁻.

By “phosphonyl” is meant —PO₃R₂, wherein each R is independent H oralkyl, as defined herein. An exemplary ion of phosphoryl is —PO₃R⁻.

By “oxo” is meant ═O.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scheme of redox potentials of interest. (a) Range of redoxpotentials spanning roughly 2V in dimethyl sulfoxide exhibited byquinones from [10]. (b) Range of aqueous standard reduction potentialsvs. SHE (pH 0).

FIG. 2 is scheme of a battery having a hydroquinone at the negativeelectrode and bromine at the positive electrode. (a) charge mode; (b)discharge mode.

FIG. 3 is a set of cyclic voltammograms (a) 1 m catechol in 1 N H₂SO₄.The plot shows the oxidative current density vs. voltage of a 0.149 cm²working electrode of flat Pt. (b) 3.9 m catechol in 1 N H₂SO₄ reached370 mA/cm² with no sign of peaking.

FIG. 4 is a half-cell cyclic voltammogram for hydroquinone sulfonicacid.

FIG. 5 is (a) Cyclic voltammogram of AQDS (1 mM) in 1 M H₂SO₄ on aglassy carbon electrode (scan rate=25 mV s⁻¹). (b) Rotating diskelectrode measurements using a glassy carbon electrode in 1 M H₂SO₄ ateleven rotation rates ranging from 200 (red) to 3600 rpm (dk. green).(c) Pourbaix diagram of AQDS. Solid lines indicate slopes of −59 mV/pH,−30 mV/pH, and 0 mV/pH, corresponding to two-, one-, and zero-protonprocesses respectively. Dashed lines linearly extrapolate the one- andzero-proton processes to give E° values of 18 mV (2e⁻/1 H⁺) and −296 mV(2e⁻/0 H⁺).

FIG. 6 is a Levich plot (current vs. rotation rate) of 1 mM AQDS in 1 MH₂SO₄. Best fit line has a slope of 0.453(2) μA s^(1/2) rad^(−1/2).

FIG. 7 is a Koutecý-Levich plot (current⁻¹ vs. rotation rate^(−1/2)).

FIG. 8 is a Tafel plot (overpotential vs. log(current)) constructedusing the current response in the absence of mass-transport at lowoverpotentials, extrapolated from the zero-intercept of FIG. 7 (infiniterotation rate). Best fit line is the function y=62(x+4.32). This yieldsα=0.474(2) and k₀=7.2(5)×10⁻³ cm s⁻¹.

FIG. 9 is a cyclic voltammogram plot of9,10-anthraquinone-2,7-disulfonic acid (AQDS) 1 mM in 1 M H₂SO₄ on aglassy carbon working electrode (black) and of anthraquinone sulfonicacid mixture solution.

FIG. 10 is a cyclic voltammograms of 9,10-anthraquione-2,7-disulfonicacid (AQDS) and 1,8-dihydroxy-9,10-anthraquinone-2,7-disulfonic acid(1,8-(OH)₂-AQDS), showing that the latter has a 95 mV lower reductionpotential.

FIG. 11 a is a scheme of p-benzoquinone as the positive material and H₂gas as the negative material for fuel cell tests. FIG. 11 b is an imageof the cell used. FIG. 11 c is a graph of cell potential versus currentdensity for tests in Example 8 using a 0.1 M solution. FIG. 11 d is agraph of the cell power density as a function of galvanic currentdensity for Example 8.

FIG. 12 is a Cell Schematic. Electrolytic/charge mode is shown; thearrows are reversed for galvanic/discharge mode.

FIG. 13 is (a) Cell potential vs. current density at five differentstates-of-charge. The inset shows a linear increase in cell potential asthe state of charge is increased. (b) Plot of galvanic (discharge) powerdensity vs. current density at the same five states of charge as (a).(c) Plot of electrolytic (charging) power density vs. current density atthe same five states of charge as (a). (d) Cell potential measured uponcycling at 500 mA cm⁻².

FIG. 14 is (a) Cell potential vs. current density at six differentstates-of-charge for the cell in Example 9. (b) Plot of power densityvs. current density at the same six states of charge as (a).

FIG. 15 is a plot of cell potential vs. state of charge for Example 9;inset shows stable current cycling over 100 shallow cycles.

FIG. 16 is a plot of cell potential vs. time from Example 11, measuredupon cycling (charge and discharge) ten times at 500 mA cm⁻².

FIG. 17 is ¹H NMR (500 MHz, D₂O) spectrum of (a) AQDS. 6=7.99 ppm (d,J=2 Hz, 1,8 C—H), 7.79 (dd, J=2 and 8 Hz, 4,5 C—H), 7.50 (d, J=8 Hz, 3,6C—H). (b) The same sample, 20 h after addition of Br₂. (c)¹H NMR of AQDStreated with 2 M HBr and Br₂ and heated to 100° C. for 48 h. The peaksare shifted due to presence of trace HBr which shifted the residualsolvent peak due to increased acidity. Coupling constants for each peakare identical to (a).

FIG. 18 is ¹³C NMR (500 MHz, D₂O) spectrum of (a) AQDS. δ=181.50 (C 9),181.30 (C 10), 148.51 (C 2,7), 133.16 (C 11), 132.40 (C 12), 130.86 (C3,6), 128.59 (C 4,5), 124.72 ppm (C 1,8). (b) The same sample, 24 hafter addition of Br₂. (c)¹³C NMR of AQDS treated with 2 M HBr and Br₂and heated to 100° C. for 48 h.

DETAILED DESCRIPTION OF THE INVENTION

The invention points the way to a high efficiency, long cycle life redoxflow battery with reasonable power cost, low energy cost, and all theenergy scaling advantages of a flow battery. In some embodiments, theseparator can be a cheap hydrocarbon instead of a fluorocarbon, andreactant crossover will be negligible. The electrodes can be inexpensiveconductors, conformally coated with a layer of active material so thinas to be negligible in cost [9]. Many of the structural components canbe made of cheap plastic, and components that need to be conducting canbe protected with conformally coated ultrathin films. Chemical storagecan be in the form of cheap, flowing liquids held in cheap plastic tanksand require neither compression nor heating above the liquid's boilingpoint. The electrochemical cells are based on small organic molecules(SOMs) called quinones (FIG. 1). Because quinone-to-hydroquinone cyclingoccurs rapidly and reversibly in photosynthesis, we are able to employit to obtain high current density (high current density is veryimportant because the cost per kW of the system is typically dominatedby the electrochemical stack's cost per kW, which is inverselyproportional to the power density—the product of current density andvoltage), high efficiency, and long lifetime in a flow battery. Thereare hundreds of different quinones spanning a wide range in properties[10-13] such as reduction potential (FIG. 1), solubility and stabilityin water and other solvents. In addition, there are many structures thatcan be readily screened computationally and synthesized. For example,quinones with high redox potential and candidates with low redoxpotential, along with other desirable attributes can be identified basedon computation screens. In one embodiment, a full cell includes a lowredox potential quinone/hydroquinone couple and a bromine/bromidecounterelectrode. In another embodiment, the full cell includes a highredox potential quinone/hydroquinone couple vs. a low redox potentialquinone/hydroquinone couple. A performance target is 80% round-tripefficiency in each cell at 0.25 W/cm².

The organic quinone species, e.g., anthraquinones, can be synthesized[39] from inexpensive commodity chemicals that cost a factor of threeless per kWh of storage than the vanadium metal ions used in the mosthighly commercialized flow battery systems. It also permits furtherorganic functionalization to increase the cell voltage and energystorage capacity. Upon scale-up, quinone-based flow batteries canprovide massive electrical energy storage at greatly reduced cost.

Small Organic Molecule (SOM) Technical Background

The invention is employs a knowledge base in oxygen-free fuel cells[14-16]. There is also a growing knowledge base on SOM electrochemistryfor hydrogen storage [17,18]. Organic-based fuel cells have been thesubject of numerous studies, many focusing on alcohols (methanol andethanol) and formic acid (H⁺COOH⁻). Cells utilizing these fuelstypically rely on high precious metal content catalysts (Pt, Pd, or Ru)[19-21]. Current densities approaching 1 A/cm² and power densitiesexceeding 250 mW/cm² have been obtained in direct formic acid fuel cells[19]. Reactant crossover is more important with methanol than formicacid [21]. Although there are a number of choices for a SOM redox couple[22-24], quinone-based compounds present a highly promising class ofSOMs. Quinones are abundant in nature, they play a vital role inoxygen-evolving photosynthesis, and we eat them in green vegetables. Inparticular, plastoquinone is reversibly and rapidly reduced toplastoquinol as part of the electron transport chain that ultimatelyleads to the reduction of NADP+ to NADPH, which is then used in thesynthesis of useful organic molecules from CO₂ [25]. A 2009 publicationexploring quinones for flow batteries makes the potential clear for flowbatteries based on quinone/hydroquinone couples [26]. They reported onepromising quinone/hydroquinone couple (sulfonic quinol) as the positiveelectrode against the conventional Pb/PbSO₄ negative solid electrode.They obtained disappointing current densities of order 10 mA/cm². Indeedthe reported [13] exchange current density is relatively high for thepara-benzoquinone/hydroquinone couple on smooth Pt. It is comparable tothat for the chlorine/chloride couple on smooth RuO₂—the basis of thecommercial Dimensionally Stabilized Anode (DSA) for the Chlor-Alkaliindustry [27].

The quinone to hydroquinone reduction reaction consists of converting anoxygen that is doubly bonded (“═O”) to an sp² C₆ ring into asingly-bonded hydroxyl (“—OH”), as shown in FIG. 2( a). An electrodecontributes an electron as the acidic electrolyte provides the proton.This typically occurs with pairs of oxygens in the ortho or paraconfigurations; in aqueous solutions the two oxygen sites undergo thereaction at potentials that are virtually indistinguishable. Thetransition from the hydroquinone to the quinone involves simply removingprotons without disrupting the rest of the bonding (FIG. 2( b)), and sothese molecules are exceedingly stable. Because the redox potentialsshift with changing solvent, but the hierarchy is much less affected,the 2-Volt range reported in dimethyl sulfoxide in FIG. 1( a) isencouraging for the prospects in aqueous electrolyte (FIG. 1( b)). Thefirst concern we have in creating a quinone-based flow battery isselecting a quinone with the appropriate value of the redox potential(FIG. 1). In aqueous solutions the positive electrode cannot operate atvoltages above about 1.5 V vs. Standard Hydrogen Electrode (SHE) or elseO₂ evolution becomes significant. The negative electrode cannot operateat voltages below about −0.2 V to 0 V (depending on electrocatalyst) vs.SHE or else H₂ evolution becomes significant. These reactions are nearthe ends of the range of potentials shown in FIG. 1( b). The survey,from which selections are shown in FIG. 1( b), is limited by somediscrepancies in reported literature values, e.g. Nivinskas et al. [28]claim a redox potential of 0.040 V for tetramethylbenzoquinone, whereasSong et al. claim 0.068 V [29]. Nevertheless, it is clear from thefigure that adding electron-withdrawing groups, such as Cl, raises theredox potential whereas adding electron-donating groups, such as methylor isopropyl, lowers the redox potential.

In addition to redox potential, important molecular characteristicsinclude solubility, stability, toxicity, and potential or current marketprice. High solubility is important because the mass transportlimitation at high current density in a full cell is directlyproportional to the solubility. Solubility can be enhanced by attachingpolar groups such as the sulfonate groups, as in1,2-Dihydroxybenzene-3,5-disulfonic acid (FIG. 1( b)). Stability isimportant not only to prevent chemical loss for long cycle life, butalso because polymerization on the electrode can compromise theelectrode's effectiveness. Stability against water and polymerizationcan be enhanced by replacing vulnerable C—H groups adjacent to C+Ogroups with more stable groups such as C—R, where R is optionallysubstituted C₁₋₆ alkyl, hydroxy, optionally substituted C₁₋₆ alkoxy,SO₃H, amino, nitro, carboxyl, phosphoryl, or phosphonyl.

Many quinones or hydroquinones are available commercially on a smallscale, and their current market price sets an upper limit on what theprice might be at large scale. The very common 1,4-parabenzoquinone(“BQ”), for example, currently costs only about $10.53/kWh, assuming a1-V cell, as shown in Table 2. Other quinones can be synthesized.

TABLE 2 Market prices of energy storage chemicals and chemicals price ofcells made up of two such chemicals. For comparison we have assumed cellvoltages of 1.2 V for vanadium and 1.0 V for all other chemicals. E_(aq)$/kWh Compound $/kg Source [V] per side Vanadium pentoxide $28.48 USGS1.2 $80.54 (V₂O₅) (cost in 2011) Benzoquinone $5.27 Shanghai Smart 1$10.63 (BQ) Chemicals Co Bromine (Br₂) $1.53 USGS 1 $4.57 (cost in 2006)Carbon Dioxide $0.73 Airgas price 1 $0.60 (CO₂) for bulk CO₂ Formic acid$1.20 DNV Risk 1 $1.03 (HCOOH) Management Firm Full Cell $/kWhBenzoquinone with Bromine $15.19 Benzoquinone with CO₂ $11.22Benzoquinone with

$21.25 Vanadium with Vanadium $161.08

indicates data missing or illegible when filed

Examples of quinones useful in the invention include those of formulas(A)-(D):

wherein each of R₁-R₁₀ is independently selected from H, optionallysubstituted C₁₋₆ alkyl, halo, hydroxy, C₁₋₆ alkoxy, SO₃H, amino, nitro,carboxyl, phosphoryl, phosphonyl, and oxo, or an ion thereof, providedthat two of R₁-R₆ for formula (A) are oxo, two or four of R₁-R₈ forformula (B) are oxo, and two, four, or six of R₁-R₁₀ for formulas (C)and (D) are oxo, wherein the dashed lines indicate that the monocylicring of formula (A), the bicyclic ring of formula (B), and the tricyclicrings of formulas (C) and (D) are fully conjugated. Typically at leastone of the R groups that is not oxo for each of formulas (A)-(D) is notH. In certain embodiments, none of the R groups for formulas (A)-(D) areH. Other formulas are (I), (II), and (III):

wherein each of R₁-R₈ is independently selected from H, C₁₋₆ alkyl(e.g., methyl, ethyl, propyl, or isopropyl), halo (e.g., F, Cl, or Br),hydroxy, C₁₋₆ alkoxy (e.g., methoxy), and SO₃H, or an ion thereof.Typically, at least one of R₁—R₈ (R₁—R₄ for (I) and (II)) is not H. Inother embodiments, none of R₁—R₈ (R₁—R₄ for (I) and (II)) is H.

Additional quinones are of any of the following formulas.

Specific examples of quinones are as follows:

Additional quinones are in the following Table 3:

Entry Name Diagram 1 9,10-anthraquinone-2,7- disulfonic acid

2 9,10-anthraquinone-2,6- disulfonic acid

3 9,10-anthraquinone-1,8- disulfonic acid

4 9,10-anthraquinone-1,5- disulfonic acid

5 9,10-anthraquinone-2- sulfonic acid

6 9,10-anthraquinone-2,3- dimethanesulfonic acid

7 1,8-dihydroxy-9,10-anthraquinone- 2,7-disulfonic acid

8 1,5-dihydroxy-9,10-anthraquinone- 2,6-disulfonic acid

9 1,4-dihydroxy-9,10-anthraquinone- 2-sulfonic acid

10 1,3,4-trihydroxy-9,10-anthraquinone- 2-sulfonic acid

11 1,2-naphthoquinone-4- sulfonic acid

12 1,4-naphthoquinone-2- sulfonic acid

13 2-chloro-1,4-naphthoquinone-3- sulfonic acid

14 2-bromo-1,4-naphthoquinone- 3-sulfonic acid

Yet further quinones are the in Table 4:

—OH ID substituted R₁ R₃ R₄ R₅ R₆ R₈ 1 Non- H H H H H H 2 OH H H H H H 3Mono- H OH H H H H 4 H H OH H H H 5 OH OH H H H H 6 OH H OH H H H 7 OH HH OH H H 8 OH H H H OH H 9 Di- OH H H H H OH 10 H OH OH H H H 11 H OH HOH H H 12 H OH H H OH H 13 H H OH OH H H 14 OH OH OH H H H 15 OH OH H OHH H 16 OH OH H H OH H 17 OH OH H H H OH 18 Tri- OH H OH OH H H 19 OH HOH H OH H 20 OH H OH H H OH 21 OH H H OH OH H 22 H OH OH OH H H 23 H OHOH H OH H 24 OH OH OH OH H H 25 OH OH OH H OH H 26 OH OH OH H H OH 27 OHOH H OH OH H 28 Tetra- OH OH H OH H OH 29 OH OH H H OH OH 30 OH H OH OHOH H 31 OH H OH OH H OH 32 H OH OH OH OH H 33 OH OH OH OH OH H 34 Penta-OH OH OH OH H OH 35 OH OH OH H OH OH 36 Hexa- OH OH OH OH OH OHQuinones or hydroquinones may be present in a mixture. For example, amixture of sulfonated quinones can be produced by reacting sulfuric acidwith an anthraquinone, e.g., 9,10-anthraquinone.

Quinones may be dissolved or suspended in aqueous solution in thebatteries. The concentration of the quinone ranges, for example, from 3M to liquid quinone, e.g., 3-15 M. In addition to water, solutions mayinclude alcohols (e.g., methyl, ethyl, or propyl) and other co-solventsto increase the solubility of a particular quinone. In some embodiments,the solution of quinone is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%,or 80% water, by mass. Alcohol or other co-solvents may be present in anamount required to result in a particular concentration of quinone. ThepH of the aqueous solution for a quinone may also be adjusted byaddition of acid or base, e.g., to aid in solubilizing a quinone.

A Quinone Flow Battery

This cell is based on a quinone/hydroquinone couple with low redoxpotential (an example of which is shown in FIG. 2) vs. redox activespecies, e.g., the bromide/bromine couple. Other redox active speciesinclude chlorine, iodine, oxygen, vanadium, chromium, cobalt, iron,manganese, cobalt, nickel, copper, or lead, e.g., a manganese oxide, acobalt oxide or a lead oxide. If the quinone redox potential is −0.02 V,then the equilibrium potential will be about 1.1 V, varying withconcentration according to the Nernst Equation. Examples ofquinone/hydroquinone couples with a low redox potential include2-Methyl-5-iso-propyl-1,4-benzoquinone or 2,6-Methoxyl-1,4-benzoquinone(FIG. 1( b)).

A high-potency low-cost chlorine/chloride and bromine/bromideelectrocatalyst is known [30], and a powerful chlorine/chloride cell hasbeen developed [9,31]. While the use of bromine is advantageous in manysystems, use in a manned environment, such as the home, is limited basedon toxicity concerns; therefore bromine-based systems are best focusedon industrial and some commercial applications. Nevertheless, thetoxicity is not so high, and its safe handling not so difficult, so asto prevent its commercialization in zinc-bromine batteries.

An all-Quinone/Hydroquinone Flow Battery.

This cell is based on the quinone/hydroquinone couple with high redoxpotential vs. quinone/hydroquinone with low redox potential. Anall-quinone cell brings many advantages. Many of the structuralcomponents could be made of cheap plastic. The molecules are big enoughthat the separator is expected to be much cheaper than Nafion [32-34],and reactant crossover will still be negligible. The electrodes can beinexpensive conductors such as titanium [35] or glassy carbon,conformally coated with layer of active material so thin as to benegligible in cost. Engineering for two-phase flow will be unnecessary.Chemical storage can be in the form of flowing liquids requiring neithercompression nor heating above the boiling point of water.

Electrode Materials

Electrode materials can be screened for good molecule-specific electrodekinetics. Although evidence indicates that quinone/hydroquinonecatalysis is not a significant barrier, some electrode materials areexpected to become deactivated due to the chemisorption of molecules orfragments, or the polymerization of reactants. Electrodes for use with aquinone or hydroquinone include any carbon electrode, e.g., carbon paperelectrodes, carbon felt electrodes, or carbon nanotube electrodes.Electrodes suitable for other redox active species are known in the art.

Fabrication of Full Cell

The fabrication of full cells requires the selection of appropriateelectrodes. Bromine and quinone electrodes can be made of a highspecific surface area conducting material, such as nanoporous metalsponge [35], which has synthesized previously by electrochemicaldealloying [36], or conducting metal oxide, which has been synthesizedby wet chemical methods and shown to be good for bromine [9,30].Chemical vapor deposition can be used for conformal coatings of complex3D electrode geometries by ultra-thin electrocatalyst films.

Fabrication of Testing Hardware and Cell Testing

The balance of system around the cell will include fluid handling andstorage, and voltage and round-trip energy efficiency measurements canbe made. Systems instrumented for measurement of catholyte and anolyteflows and pH, pressure, temperature, current density and cell voltagemay be included and used to evaluate cells. Testing can be performed asreactant and acid concentrations and the cell temperature are varied. Inone series of tests, the current density is measured at which thevoltage efficiency drops to 90%. In another, the round-trip efficiencyis evaluated by charging and discharging the same number of amp-minuteswhile tracking the voltage in order to determine the energy conversionefficiency. This is done initially at low current density, and thecurrent density is then systematically increased until the round-tripefficiency drops below 80%. Fluids sample ports can be provided topermit sampling of both electrolytes, which will allow for theevaluation of parasitic losses due to reactant crossover or sidereactions. Electrolytes can be sampled and analyzed with InductivelyCoupled Plasma Mass Spectrometry, and other standard techniques.

Ion Conducting Barriers

The ion conducting barrier allows the passage of protons but not asignificant amount of the quinone, hydroquinone, or other redox activespecies. Example ion conducting barriers are Nafion, i.e., sulfonatedtetrafluoroethylene based fluoropolymer-copolymer, hydrocarbons, e.g.,polyethylene, and size exclusion barriers, e.g., ultrafiltration ordialysis membranes with a molecular weight cut off of 100, 250, 500, or1,000 Da. For size exclusion membranes, the molecular weight cut offwill be determined based on the molecular weight of the quinone,hydroquinone, or other redox active species employed.

Additional Components

A battery of the invention may include additional components as is knownin the art. Quinones, hydroquinones, and other redox active speciesdissolved or suspended in aqueous solution will be housed in a suitablereservoir. A battery may further include pumps to pump aqueous solutionsor suspensions past one or both electrodes. Alternatively, theelectrodes may be placed in a reservoir that is stirred or in which thesolution or suspension is recirculated by any other method, e.g.,convection, sonication, etc. Batteries may also include graphite flowplates and aluminum current collectors.

EXAMPLES Example 1

1 molal 1,2-ortho-benzohydroquinone (catechol) was oxidized in 1 N H₂SO₄at a flat Pt electrode, obtaining the cyclic voltammetry curves shown inFIG. 3 a. The sweep starts at (0.2 V, 0 mA/cm²) and proceeds at 25 mV/sto the right. At about 600 mV vs. Ag/AgCl (the known E⁰ is 795 mV vs.SHE), the current density increases as catechol is oxidized to theorthoquinone form. The oxidative current density peaks at about 150mA/cm². The peak and downturn are caused by reactant depletion in aquiescent (non-flowing, non-stirred) electrolyte. In a test at a higherconcentration of 3.9 molal (FIG. 3 b), we observe asymmetric oxidationand reduction peaks, achieving current densities above 500 mA/cm² forthe former. The asymmetric shape of the curve in FIG. 3 b arises becausethe quinone form is unstable in aqueous solution. In addition thelimited solubility of ortho-benzoquinone (0.06 M) compared to itsreduced form precludes symmetric behavior at high concentration.

Example 2

The half-cell redox behavior of hydroquinone-2-sulfonic acid (HQSA) isshown in FIG. 4. At a pH of 7 a rise in current density was observedbeginning near 0.5 V and peaking at higher voltage. Upon reversing thedirection of the voltage sweep, negative current (indicating a reductionevent) was observed near 0.3 V. The large difference between where theoxidation and reduction currents are observed indicates a chemicalprocess was likely occurring. In this case, upon oxidation of HQSA tothe quinone form, water reacted with the quinone to form a new species.This species was reduced at the lower 0.3 V potential. At a pH of 13,the reaction became rapid and reversible because in basic solution the—OH groups on HQSA became deprotonated. The positive and negativecurrent density observed near 0 V was indicative of a 2-electron redoxevent with no protons having been exchanged.

Example 3

AQDS was subjected to half-cell electrochemical measurements. Cyclicvoltammetry of a 1 mM solution of AQDS in 1 M sulfuric acid on a glassycarbon disc working electrode showed current peaks corresponding toreduction and oxidation of the anthraquinone species (FIG. 5 a).

The peak separation of 34 mV was close to the 59 mV/n, where n was thenumber of electrons involved, expected for a two-electron process.

Example 4

The glassy carbon disk in Example 3 was rotated at a variety of ratesyields mass-transport limited currents from which the AQDS diffusioncoefficient (D=3.8(1)×10⁻⁶ cm² s⁻¹) (compare D in [38]) and kineticreduction rate constant could be determined (FIGS. 5 b, 6, 7, and 8).Kinetic data showed the rate constant for AQDS reduction on glassycarbon to be k₀=7.2(5)×10⁻³ cm s⁻¹, which exceeded the rate constant onAu [39]. This rate constant was faster than that found for many otherspecies used in flow cells such as V³⁺/V²⁺, Br₂/Br⁻, and S₄ ²/S₂ ²⁻ (seeTable 2 in [40]). The electrochemical reversibility of the two-electronredox reaction was confirmed by the slope of the Tafel plot (FIG. 8),which gave the transfer coefficient α=0.474, which is close to the valueof 0.5 expected for an ideally reversible reaction.

Example 5

To further understand the AQDS redox behavior, we generated a Pourbaixdiagram (FIG. 5 c) of the equilibrium potential of the AQDS redox couplevs. pH. Aqueous 1 mM solutions of AQDS disodium salt were prepared andpH buffered using the following chemicals: sulfuric acid (1 M, pH 0),HSO₄ ⁻/SO₄ ²⁻ (0.1 M, pH 1-2), AcOH/AcO⁻ (0.1 M, pH 2.65-5), H₂PO₄⁻/HPO₄ ²⁻ (0.1 M, pH 5.3-8), HPO₄ ²⁻/PO₄ ³⁻ (0.1 M, pH 9.28-11.52), andKOH (0.1 M, pH 13). The pH of each solution was adjusted with 1 M H₂SO₄or 0.1 M KOH solutions. In acidic solutions (pH<7), the 59 mV/pH slopeindicated that a two-electron, two-proton process occurs ([39]). In morebasic conditions (7<pH<11), a two-electron, one-proton process occurred,giving a 30 mV/pH slope. The potential became pH-independent at valuesgreater than 11, which indicated a two-electron, zero-proton process.These results indicated that AQDS performed reversible two-electronredox chemistry in a pH range of 0 to 14, and the protonation state ofthe reduction product dihydro-AQDS, which yielded approximate pK_(a)values of 7 and 11.

Example 6

A solution of anthraquinone was heated in concentrated sulfuric acid ora solution of 30% SO₃ in concentrated sulfuric acid (oleum), resultingin a mixture of sulfonated anthraquinones as previously described [37].This crude mixture was allowed to cool to room temperature and wasdiluted with 1 M sulfuric acid to give a solution of 1 mM sulfonatedanthraquinone. This solution was subjected to half-cell measurementsthat demonstrate that the behavior of the mixture of sulfonatedanthraquinones was nearly identical to the pure9,10-anthraquinone-2,7-disulfonic acid illustrated in Example 3, asshown in FIG. 9.

Example 7

A solution of 1,8-dihydroxy-9,10-anthraquinone was heated inconcentrated sulfuric acid and a yellow solid was isolated afteraddition of NaCl, which contained1,8-dihydroxy-9,10-anthraquinone-2,7-disulfonic acid disodium salt(1,8-(OH)₂-AQDS) in >95% purity. A solution consisting of 1 mM1,8-(OH)₂-AQDS in 1 M H₂SO₄ was subjected to half-cell measurementsunder similar conditions to Example 3. The traces of 1,8-(OH)₂-AQDS andAQDS are shown in FIG. 10, and illustrate that the peak potentials ofAQDS were shifted by nearly 100 mV by the addition of —OH groups to theAQDS backbone.

Example 8

A quinone-hydrogen fuel cell is illustrated by schematic in FIG. 11 a.1,4-benzoquinone was used as the positive electrode material and H₂ gasas the negative material for fuel cell tests. We operated the cell indischarge mode, with the p-benzoquinone/p-hydroquinone mixturerecirculated past the quinone electrode on the positive side, and H₂flowing past the hydrogen electrode. The Nafion membrane conducted H⁺ions towards the cathode. The cell reached current densities of about150 mA/cm² and power densities of about 35 mW/cm², which were higherthan values previously reported using soluble quinones for the positiveelectrode in a full cell configuration [26]. We used a fuel cell testbench constructed by Sustainable Innovations, LLC, and modified in ourlab [9]. FIG. 11 b shows an image of the cell used. The cell featuredaluminum endplates, pyrolytic graphite current collectors withserpentine flow channels, a 50 μm thick Nafion 212 proton exchangemembrane (which prior to use was pretreated using methods previouslydescribed [9]), and PTFE/Viton tubing and gasketing throughout. On bothsides of the cell, a commercial Pt—Ru/C carbon paper commercialelectrode was used. The cell was operated in galvanic mode usingpreviously described methods [9], with high-purity hydrogen gas flowedthrough the negative side of the cell at 5 psig and quinone solutionflowed through the positive side using a Cole Parmer Masterflex pump.The solution consisted of para-benzoquinone in 1 N H₂SO₄. Before eachset of measurements, an N₂ purge was performed to remove any remainingO₂ and to ensure there were no leaks in the assembly. After reactantintroduction to the cell, the voltage was allowed to stabilize for a fewminutes, after which a DC electronic load was used to draw incrementallyhigher currents from the cell. In general, in order to allow the voltageto stabilize, we waited about 15 seconds after each change in current.In FIG. 11 c, we show the cell potential versus current density fortests done using a 0.1 M solution. In general, we observed a nearlylinear drop in potential with increasing current density indicatingrobust electrode kinetics for the redox reaction, i.e. relatively lowactivation overpotentials. In FIG. 11 d, we show the cell power densityas a function of galvanic current density. The power density fell offrapidly near the limiting current density.

Example 9

Solutions of 9,10-anthraquinone-2,7-disulfonic acid disodium salt andHBr in 1 M sulfuric acid were pumped through a flow cell as outlined inFIG. 12. Circular endplates were machined out of solid aluminum. 3 in.×3in. pyrolytic graphite blocks with single-serpentine flow channels(channel width=0.0625 in., channel depth=0.08 in., landing betweenchannels=0.031 in., Fuel Cell Technologies, Inc.) were used as currentcollectors. Pretreated 2 cm², double-stacked Toray carbon paperelectrodes (each of which is about 7.5 μm uncompressed) were used onboth sides of the cell. Pretreatment consisted of a 10 min. sonicationin isopropyl alcohol followed by a 30 min. immersion in hot (80° C.) 6 Msulfuric acid and then a 4 hr. heat treatment in an air furnace at 400°C. Nafion® 212 (50 μm thick) was used as a proton-exchange membrane(PEM, Alfa Aesar), and PTFE gasketing was used to seal the cellassembly. Membrane pretreatment was done according to previouslypublished protocols [9]. Six bolts (⅜″-16) torqued to 10.2 Nm completedthe cell assembly, and PTFE tubing was used to transport reactants andproducts into and out of the cell. The cell was kept on a hot plate andwrapped in a PID-controlled heating element for temperature control, andthe liquid electrolyte reservoirs were heated to improve thermalmanagement. On the positive side of the cell, 35 mL of 1.75 M HBr and0.9375 M NaHSO₄ were used as the electrolyte solution. On the negativeside, 0.75 M 2,7-AQDS disodium salt in 1 M H₂SO₄ were used. Theseconcentrations were used so that, at a 50% state of charge, no pH ortotal ion concentration gradients exist across the membrane.Measurements shown here were done at 50° C. A Masterflex® peristalticpump was used to circulate the fluids. A CHInstruments 1100Cpotentiostat was used to measure electrochemical properties of thebattery. A potential of 1.5 volts was applied to charge the cell. Thepotential-current response (FIG. 14 a), potential-power (FIG. 14 b), andopen circuit potential (FIG. 15) for various states of charge (SOCs)were measured. As the SOC increased from 20% to 90%, the open circuitpotential increased linearly from 0.76 V at 0.98 V. In the galvanicdirection, peak power densities were 77 mW cm⁻² and 168 mW cm⁻² at thesesame SOCs, respectively (FIG. 14 b). In order to avoid significant watersplitting in the electrolytic direction, we used a cut-off voltage of1.5 V, at which point the current densities observed at 20% and 90% SOCswere −630 mA cm⁻² and −196 mA cm⁻², respectively, with correspondingpower densities of −939 mW cm⁻² and −291 mW cm⁻². As an investigation ofthe reproducibility and durability of the QBFB, the voltage was cycled±0.6 V away from the open circuit potential (0.85 V @ 50% SOC) onehundred times for one minute each. The current density at the end ofeach cycle (FIG. 15, inset) was constant over the time scale of theexperiment, and indicated that there were no immediate degradation,fouling, or crossover issues in the cell.

Example 10

Performance characteristics of a quinone-bromine flow battery weremeasured under identical conditions to Example 9, except for thefollowing: A 0.1 M solution of 9,10-anthraquinone-2,7-disulfonic acid in1 M sulfuric acid was used for the negative electrolyte solution; 0.2 MHBr in 1 M sulfuric acid was used as the positive electrolyte solution;Interdigitated flow channels (channel width=0.0625 in., channeldepth=0.08 in., landing between channels=0.031 in., Fuel CellTechnologies, Inc.) were used as current collectors; Pretreated 2 cm²,stacked (6×) Toray carbon paper electrodes (each of which is about 7.5μm uncompressed) were used on both sides of the cell—pretreatmentconsisted of a 10 minute sonication in isopropyl alcohol followed by afive hour soak in a hot (50° C.) mixture of undiluted sulfuric andnitric acids in a 3:1 volumetric ratio. Constant-current cycling datawere collected at 0.2 A cm⁻². The cycles were highly reproducible andindicate that columbic efficiencies for the battery were, at a minimum,around 95% (FIG. 13 d).

Example 11

Performance characteristics of a quinone-bromine flow battery weremeasured under identical conditions to Example 10, except for thefollowing: 120 mL of 2 M HBr and 0.5 M Br₂ were used as the positiveelectrolyte solution; 1 M 2,7-AQDS in 2 M H₂SO₄ was used as the negativeelectrolyte solution. As the SOC increased from 10% to 90%, the opencircuit potential increased linearly from 0.69 V to 0.92 V (FIG. 13 a,inset). In the galvanic direction, peak power densities were 0.246 Wcm⁻² and 0.600 W cm⁻² at these same SOCs, respectively (FIG. 13 b). Inorder to avoid significant water splitting in the electrolyticdirection, we used a cut-off voltage of 1.5 V, at which point thecurrent densities observed at 10% and 90% SOCs were −2.25 A cm⁻² and−0.95 A cm⁻², respectively, with corresponding power densities of −3.342W cm⁻² and −1.414 W cm². The cell was cycled at 500 mA cm⁻², and thevoltage was recorded (FIG. 16). This showed a coulombic efficiency ofover 93% and no loss in charge capacity over the course of 10 cycles and100 hours.

Example 12

50 mg of 9,10-anthraquinone-2,7-disulfonic acid was dissolved in 0.4 mLof D₂O was treated with 100 μL of Br₂. The ¹H and ¹³C NMR spectra (FIGS.17 and 18, a and b) were unchanged from the starting material afterstanding for 20 hours at 25° C. 50 mg of AQDS was then treated with 1 mLof concentrated HBr and 100 μL of Br₂. The reaction was heated to 100°C. for 48 h and evaporated to dryness at that temperature. The resultingsolid was fully dissolved in D₂O giving unchanged ¹H and ¹³C NMR (FIGS.17 and 18, c); however, the ¹H NMR reference was shifted due to residualacid. 9,10-anthraquinone-2,7-disulfonic acid demonstrated no reactionwith 2 M HBr and bromine when heated to 100° C. for two days (FIGS. 17and 18), meaning that bromine crossover will not lead to irreversibledestruction of AQDS.

Example 13

1 mM solutions of the quinones listed in the following table wereprepared in 1 M sulfuric acid solution. The pH of the solutions was 0.Half-cell electrochemical data were recorded using a working electrodeconsisting of a flat 3 mm diameter disk of glassy carbon, a coiledplatinum wire as a counter electrode, and a Ag/AgCl reference electrode.Cyclic voltammograms were recorded using sweep rates of 25 mV/s. The E°was measured for each quinone by taking the average voltage value of theanodic and cathodic current density peaks and adding 0.210 V to convertform the Ag/AgCl reference to the standard hydrogen electrode (SHE).

Standard Reduction Potential E⁰ in Volts vs. the standard Entry NameDiagram hydrogen electrode (SHE) 1 9,10-anthraquinone- 2,7-disulfonicacid

0.213 2 9,10-anthraquinone- 2,6-disulfonic acid

0.212 3 9,10-anthraquinone- 1,8-disulfonic acid

0.182 4 9,10-anthraquinone- 1,5-disulfonic acid

0.223 5 9,10-anthraquinone- 2-sulfonic acid

0.171 6 9,10-anthraquinone- 2,3-dimethanesulfonic acid

0.114 7 1,8-dihydroxy-9,10- anthraquinone-2,7- disulfonic acid

0.118 8 1,5-dihydroxy-9,10- anthraquinone-2,6- disulfonic acid

0.116 9 1,4-dihydroxy-9,10- anthraquinone-2- sulfonic acid

0.094 10 1,3,4-trihydroxy-9,10- anthraquinone-2- sulfonic acid

0.088 11 1,2-naphthoquinone- 4-sulfonic acid

0.423 12 1,4-naphthoquinone- 2-sulfonic acid

0.356 13 2-chloro-1,4- naphthoquinone- 3-sulfonic acid

0.368 14 2-bromo-1,4- naphthoquinone- 3-sulfonic acid

0.371

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1. A rechargeable battery comprising first and second electrodes,wherein in its charged state, the battery comprises a redox activespecies in contact with the first electrode and a hydroquinone dissolvedor suspended in aqueous solution in contact with the second electrode,wherein during discharge the redox active species is reduced and thehydroquinone is oxidized to a quinone.
 2. The rechargeable battery ofclaim 1, wherein the redox active species is dissolved or suspended inaqueous solution.
 3. The rechargeable battery of claim 1, wherein theredox active species in contact with the first electrode compriseschlorine, bromine, iodine, oxygen, vanadium, chromium, cobalt, iron,manganese, cobalt, nickel, copper, or lead.
 4. The rechargeable batteryof claim 1, wherein the redox active species in contact with the firstelectrode comprises bromine.
 5. The rechargeable battery of claim 3,wherein the redox active species in contact with the first electrodecomprises a manganese oxide, a cobalt oxide or a lead oxide.
 6. Therechargeable battery of any of claims 1-5, wherein the hydroquinone andquinone in contact with the second electrode have a standardelectrochemical potential below 0.4 volts with respect to a standardhydrogen electrode.
 7. The rechargeable battery of claim 6, wherein thequinone is a water-soluble anthraquinone.
 8. The rechargeable battery ofclaim 7, wherein the water-soluble anthraquinone comprises one or moresulfonate groups.
 9. The rechargeable battery of claim 8, wherein theanthraquinone is 9,10-anthraquinone-2,7-disulfonate.
 10. Therechargeable battery of any of claims 6-9, wherein first and secondelectrodes are separated by an ion conducting barrier, and the redoxactive species comprises bromine.
 11. The rechargeable battery of claim1, wherein the redox active species is a second quinone dissolved orsuspended in aqueous solution.
 12. The rechargeable battery of claim 11,wherein the first electrode is separated from the second electrode by abarrier that inhibits the passage of the redox-active species and thehydroquinone.
 13. The rechargeable battery of claim 12, wherein thebarrier is a size exclusion barrier.
 14. A rechargeable batterycomprising first and second electrodes separated by an ion conductinghydrocarbon barrier or size-exclusion barrier, wherein in its chargedstate, the battery comprises a quinone at the first electrode and ahydroquinone at the second electrode, wherein during discharge, thequinone is reduced, and the hydroquinone is oxidized.
 15. Therechargeable battery of any of claims 1-5 and 11-14, wherein the quinoneor hydroquinone in oxidized form is of formula (A)-(D):

wherein each of R₁-R₁₀ is independently selected from H, optionallysubstituted C₁₋₆ alkyl, halo, hydroxy, optionally substituted C₁₋₆alkoxy, SO₃H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo,or an ion thereof, provided that two of R₁-R₆ for formula (A) are oxo,two or four of R₁-R₈ for formula (B) are oxo, and two, four, or six ofR₁-R₁₀ for formulas (C) and (D) are oxo, wherein the dashed linesindicate that the monocylic ring of formula (A), the bicyclic ring offormula (B), and the tricyclic rings of formulas (C) and (D) are fullyconjugated.
 16. The rechargeable battery of any of claims 1-5 and 11-14,wherein the quinone or hydroquinone in oxidized form is of formula(I)-(IX):

wherein each of R₁-R₈ is independently selected from H, optionallysubstituted C₁₋₆ alkyl, halo, hydroxy, optionally substituted C₁₋₆alkoxy, SO₃H, amino, nitro, carboxyl, phosphoryl, phosphonyl, and oxo,or an ion thereof.
 17. The rechargeable battery of any of claims 1-5 and11-14, wherein the quinone or hydroquinone in oxidized form is:9,10-anthraquinone-2,7-disulfonic acid,9,10-anthraquinone-2,6-disulfonic acid,9,10-anthraquinone-1,8-disulfonic acid,9,10-anthraquinone-1,5-disulfonic acid, 9,10-anthraquinone-2-sulfonicacid, 9,10-anthraquinone-2,3-dimethanesulfonic acid,1,8-dihydroxy-9,10-anthraquinone-2,7-disulfonic acid,1,5-dihydroxy-9,10-anthraquinone-2,6-disulfonic acid,1,4-dihydroxy-9,10-anthraquinone-2-sulfonic acid,1,3,4-trihydroxy-9,10-anthraquinone-2-sulfonic acid,1,2-naphthoquinone-4-sulfonic acid, 1,4-naphthoquinone-2-sulfonic acid,2-chloro-1,4-naphthoquinone-3-sulfonic acid,2-bromo-1,4-naphthoquinone-3-sulfonic acid, or a mixture thereof. 18.The rechargeable battery of claim 17, wherein the quinone orhydroquinone in oxidized form is: 9,10-anthraquinone-2,7-disulfonicacid, 9,10-anthraquinone-2,6-disulfonic acid,9,10-anthraquinone-1,8-disulfonic acid,9,10-anthraquinone-1,5-disulfonic acid, 9,10-anthraquinone-2-sulfonicacid, or a mixture thereof.
 19. The rechargeable battery of any ofclaims 1-5 and 11-14, wherein the quinone or hydroquinone in oxidizedform is: 2-hydroxy-1,4-naphthoquinone-3-sulfonic acid,1,2,4-trihydroxybenzene-3-sulfonic acid,2,4,5-trihydroxybenzene-1,3-disulfonic acid2,3,5-trihydroxybenzene-1,4-disulfonic acid,2,4,5,6-tetrahydroxybenzene-1,3-disulfonic acid,2,3,5-trihydroxybenzene-1,4-disulfonic acid,2,3,5,6-tetrahydroxybenzene-1,4-disulfonic acid, or a mixture thereof.20. The rechargeable battery of any of claims 1-19, further comprising areservoir for quinone and/or hydroquinone dissolved or suspended inaqueous solution and a mechanism to circulate quinone and/orhydroquinone.